1. Field of the Invention
The present invention relates to an optical encoder for use in detecting the position, speed, acceleration or the like of a moving or rotating object.
2. Description of the Prior Art
Japanese Patent Laid-open Application (unexamined) No. 1-199115 discloses an optical encoder as shown in FIG. 1. This optical encoder comprises a light source 1, a collimator lens 2 for converting a light emitted from the light source 1 into a parallel light, a rotary disc 3 having a plurality of slits formed therein, a fixed mask 4 having two slits formed therein, two light receiving elements 5 for receiving the light emitted from the light source 1, and two waveform shapers 6 for effecting a waveform shaping with respect to respective waveforms sent from the light receiving elements 5.
A light flux emitted from the light source 1 is initially converted into a parallel light by the collimator lens 2. The parallel light is then applied to the rotary disc 3. Part of the parallel light passes through the slits of the rotary disc 3 and those of the fixed mask 4 and is received by the light receiving elements 5. The light received by the light receiving elements 5 is converted into electric energy, which is in turn inputted into the waveform shapers 6 and is outputted therefrom as electric signals.
Upon rotation of the rotary disc 3, the overlapping condition between the slits of the rotary disc 3 and those of the fixed mask 4 gradually changes between the condition in which both of them almost overlap each other and the condition in which there is little overlap between them, as viewed from the light source 1. In compliance with the overlapping condition, the waveform shapers 6 output respective signals each generally in the form of a sine wave. Based on these signals, the rotational position, speed, acceleration or the like can be detected.
In the above-described construction, however, the pitch between the slits of the rotary disc 3 must be extremely narrowed for a detection of high resolving power.
Furthermore, the assembling of these components requires the positioning between the slits of the rotary disc 3 and those of the fixed mask 4 and the positioning between the slits of the fixed mask 4 and the light receiving elements 5. These two positionings require a high accuracy of the components and that for adjustments thereof during the assembling.
In addition, two kinds of slit plates i.e., the rotary disc 3 and the fixed mask 4 are required as an optical modulator. Since the two slit plates must be arranged in the direction axially of the optical encoder, it is difficult to make the encoder thin.